[1.1] Changing time signature should change autobeam properties

1. Create a score in 3/4 with the new score wizard
2. Enter a couple of 8th notes
3. There are beamed by 2. Delete them all
4. Change time signature to 6/8 by drag and dropping it from the palette
5. Enter some 8th notes

Expected result : notes are beamed by 3.
Actual result : notes are beamed by 2 like in 3/4.

Workaround : create a 6/8 score with the new score wizard and copy paste the music in it.

To be checked in the trunk.